The binding of Mg2+ to albumin and its relationship to Ca2+ was studied using a continuous flow dialysis technique developed in the laboratory. At high concentrations of Mg2+, a negative correlation exists between the concentrations of Mg2+ and Ca2+, but at low concentrations of Mg2+, the converse is true. The use of Mg2+ binding dyes, eriochrome blue SE and eriochrome black T, was investigated.